Aurora Season Begins

Aurora Season Begins

Autumn has arrived and that means it's time to watch
out for Northern Lights.

Sept.
23, 2002: "The kids started screaming when the sky changed.
Even the teens were excited," says Rick Thayer of Hadley,
Massachusetts. He and his family (including 6 teenagers from
a nearby birthday party) were outdoors at sunset on Sept 7th
when a solar wind gust hit Earth's magnetic field. The impact
triggered colorful Northern Lights that people saw all the way
from Europe to the western US.

"It was amazing," agrees sky watcher Brett
Clapper. He was at a star party in North Carolina when the sky
unexpectedly turned red. "A local Boy Scout troop was there
and many of the boys had never even seen the aurora borealis
before--what a treat!"

Across the Atlantic Ocean in Finland, photographer Jorma Koski
says the "the auroras were so intense, they cast shadows
on the ground."

It was a good time to be outside.

More good times are coming, say researchers, because autumn
(which begins today in the northern hemisphere) is "aurora
season." Autumn is special in part because lengthening nights
and crisp pleasant evenings tempt stargazers outside; they see
things they ordinarily wouldn't. But there's more to it than
that: autumn really does produce a surplus of geomagnetic storms--almost
twice the annual average.

In
fact, both spring and autumn are good aurora seasons. Winter
and summer are poor. This is a puzzle for researchers because
auroras are triggered by solar activity. The Sun doesn't know
what season it is on Earth--so how could one season yield more
auroras than another?

Left: Geomagnetic activity from 1875 to 1927, from
"Semiannual Variation of Geomagnetic Activity" by C.T.
Russell and R.L. McPherron, JGR, 78(1), 92, 1973. See
also a more recent
analysis by NASA solar physicist David Hathaway.

To understand the answer, we must first understand what causes
auroras themselves.

Auroras appear during geomagnetic storms--that is, when Earth's
magnetic field is vibrating in response to a solar wind gust.
Such gusts pose no danger to people on the ground because our
magnetic field forms a bubble around Earth called the magnetosphere,
which protects us. The magnetosphere is filled with electrons
and protons. "When a solar wind gust hits the magnetosphere,
the impact knocks loose some of those trapped particles,"
explains space physicist Tony Lui of Johns Hopkins University.
"They rain down on Earth's atmosphere and cause the air
to glow where they hit--like the picture
tube of a color TV."

Below: Still frames from a digital movie show how solar
wind gusts rattle Earth's magnetosphere and trigger auroras.
Click to view the 750
kb Quicktime animation created by Digital
Radiance, Inc.

Some solar wind gusts ("coronal mass ejections")
are caused by explosions near sunspots, others are caused by
holes in the Sun's atmosphere ("coronal holes") that
spew solar wind streams into interplanetary space. These gusts
sweep past Earth year-round, which returns us to the original
question: why do auroras appear more often during spring and
autumn?

The answer probably involves the Sun's magnetic field near
Earth. The Sun is a huge magnet, and all the planets in the solar
system orbit within the Sun's cavernous magnetosphere. Earth's
magnetosphere, which spans about 50,000 km from side to side,
is tiny compared to the Sun's.

The outer boundary of Earth's magnetosphere is called the
magnetopause--that's
where Earth's magnetic field bumps into the Sun's and fends off
the solar wind. Earth's magnetic field points north at the magnetopause.
If the Sun's magnetic field tilts south near the magnetopause,
it can partially cancel Earth's magnetic field at the point of
contact.

"At
such times the two fields (Earth's and the Sun's) link up,"
says Christopher Russell, a Professor of Geophysics and Space
Physics at UCLA. "You can then follow a magnetic field line
from Earth directly into the solar wind." Researchers call
the north-south component of the Sun's nearby magnetic field
"Bz" (pronounced "Bee-sub-Zee").
Negative (south-pointing) Bz's open a door through
which energy from the solar wind can reach Earth's inner magnetosphere.
Positive (north-pointing) Bz's close the door.

In the early 1970's Russell and colleague R. L. McPherron
recognized a connection between Bz and Earth's changing
seasons. "It's a matter of geometry," explains Russell.
Bz is the component of the Sun's magnetic field near
Earth which is parallel to Earth's magnetic axis. As viewed from
the Sun, Earth's tilted axis seem to wobble slowly back and forth
with a one-year period. The wobbling motion is what makes Bz
wax and wane in synch with the seasons.

In fact, Bz is always fluttering back and forth
between north and south as tangled knots of solar magnetic field
drift by Earth. What Russell and McPherron realized is that the
average size of the flutter is greatest in spring and fall. When
Bz turns south during one of those two seasons, it
really turns south and "opens the door wide"
for the solar wind.

Mystery solved? Not yet. In a recent Geophysical Research
Letter (28, 2353-2356, June15, 2001), Lyatsky et al
argued that Bz and other known effects account for
less than one-third of the seasonal ups-and-downs of geomagnetic
storms. "This is an area of active research," remarks
Lui. "We still don't have all the answers because it's a
complicated problem."

Editor's note: Seasons are reversed in Earth's two
hemispheres. Today is the beginning of both northern autumn and
southern spring. Because geomagnetic activity is higher during
spring and autumn, aurora season is therefore beginning in both
hemispheres.

Grab your camera! Just because you can't see
auroras doesn't mean they're not there. If a geomagnetic storm
is in progress (sign up for alerts from spaceweather.com), take
your camera, load it with sensitive film, face north, and take
a 30 or so second exposure. You might be surprised by what the
print reveals.

Above: Lyndon
Anderson, an experienced aurora photographer in North Dakota,
snapped the above photo on just such a night (Feb. 21, 2002).
"I recall seeing very little if anything in the northern
sky that night," he says." I took a two minute exposure,
and the photograph shows a bright aurora." Anderson notes
that when he photographs faint auroras he normally uses a 28
mm lens w 1.8 aperture, Fuji Superia Xtra 800 film, and
exposure times ranging from 30 to 45 seconds.